As a conventional technology, a method has been proposed that determines a deterioration state of an assembled battery by detecting the maximum and the minimum discharge voltages of each unit battery constituting the assembled battery at a setup time, calculating the difference between the maximum and the minimum values, and comparing the difference with a threshold. (for example, refer to Patent Document 1)
Also, for a power storage device in which equalization processing is conducted in order to control deterioration in a product life time of the storage batteries, a method has been proposed that determines, when an interval between equalization processing is equal to or shorter than a predetermined value, that the power storage device has an indication of abnormality and determines, when the total charge and discharge capacity of the power storage device determined as that having an indication of abnormality is equal to or larger than a reference value, that the power storage device has transitioned from “an almost abnormal state” to “a completely abnormal state”. (for example, refer to Patent Document 2)